Some specific spaces or rooms such as at a hospital require a proper negative or positive pressure in the interior space or room in order to prevent germs or virus dissipating through a fissure by airflow to an exterior space outside the room such as an exterior room, hallway or corridor. The interior air pressure is lower than that at the exterior because airflow travels from a space of higher pressure to a space of lower pressure. It is well known to provide a device that keeps the interior air pressure lower than that of the exterior in order to maintain a clean and healthy environment, such a system is described in U.S. Pat. No. 5,228,306. It is necessary for the doctors, nurses, and other concerned employees and individuals to easily monitor whether or not a proper vacuum (a negative pressure differential) is being maintained in a room from outside the room such as in the hallway or corridor. Fairly sophisticated devices, usually electronically controlled, have been developed for this purpose. However, these devices are expensive to procure and maintain and subject to failure. Furthermore, because the pressure differential across such walls, particularly hospital walls, is very small, present day devices are expensive and difficult to maintain and calibrate.
A pressure differential indicating apparatus, which incorporates a preferably adjustable airflow duct across the wall between the low and high pressure spaces and a low airflow indicator on the side of the wall opposite the space that is to be monitored to indicate air pressure differentials and particularly low pressure differentials across walls is disclosed in U.S. Pat. No. 5,589,643 which issued Dec. 31, 1996 and is incorporated herein by reference in its entirety.
One conventional air velocity measuring device uses a hot wire anemometer, which requires electrical power to operate and is expensive. A relatively less expensive mechanical device, a rotational vane type that has a number of intricate moving parts that require maintenance, is still relatively expensive and is not easy to monitor by a person walking down a hospital hall.
When the pressure differential indicating apparatus referenced U.S. Pat. No. 5,589,643 is placed in a fire rated wall, such as found in many hospitals and other buildings, it too must be fire rated. One method of providing a fire rated pressure differential indicating apparatus is to incorporate an intumescent material in the apparatus. Another method is to provide a door that is typically in the closed position and is opened to take a pressure differential measurement.